1. Field of the Invention
The present invention relates to an image forming apparatus, which uses an electrophotographic system or an electrostatic recording system, such as a printer, a facsimile apparatus, or a copying machine, and a developing apparatus which is preferably used in the image forming apparatus. More specifically, the present invention relates to a developing apparatus and an image forming apparatus which can perform development by selectively using a single developer out of plural kinds of developers with one developer carrying member.
2. Related Background Art
Conventionally, for example, in a typical arrangement for a color image forming apparatus using an electrophotographic system, developing apparatuses are provided in the same number as kinds (number of colors) of developers (toners) to be used, and electrostatic latent images corresponding to the respective kinds of developer (toners of the respective colors) are developed, whereby a developer image of desired colors or desired number of colors is obtained. In particular, in order to reproduce a full-color image, an image forming apparatus using developers of four different colors, namely, cyan, magenta, yellow and black, has been put to practical use. In addition, in a single color image forming apparatus, a method of using plural kinds of developers having different concentrations to enhance gradation property in representing the tone has also been proposed.
Such an image forming apparatus, which uses plural kinds of developers, represented by a color image forming apparatus requires developing apparatuses of respective colors. Thus, the image forming apparatus has problems in that, for example, a volume of the entire apparatus is increased, a large number of control high-voltage power supplies are required, control is complicated, a driving torque is increased due to an increase in the number of movable components, and power consumption is increased following the increase in the driving torque.
In order to cope with these problems resulted from the large number of developing apparatuses, there has been devised a developing apparatus which selectively furnishes plural kinds of developers for development with one developing apparatus.
As a method of separating the plural kinds of developers in such a developing apparatus, in particular, there have been proposed many systems for mixing two different kinds of developers with positive and negative charging polarities to develop two kinds of developer images with one developing apparatus (hereinafter referred to as “negative/positive mixing systems”) for their readiness of separating colors. Among the negative/positive mixing systems, the following systems are noted:
(1) a system for mixing two kinds of magnetic single-component developers containing a magnetic substance as a developer as disclosed in Japanese Patent Publication No. H5-80671;
(2) a magnetic/nonmagnetic mixing system for containing a magnetic substance in one developer and making the other developer nonmagnetic as disclosed in Japanese Patent Application Laid-Open No. S59-58442;
(3) a system for mixing two kinds of non-magnetic single-component developers as disclosed in Japanese Patent Application Laid-Open No. S55-43533; and
(4) a system for mixing a magnetic carrier in a mixed developer consisting of toners of both positive and negative polarities as disclosed in Japanese Patent Application Laid-Open Nos. S60-131554 and S63-23175.
However, in the above-described systems using the magnetic developer, it is difficult to manufacture developers having high chroma of colors other than black (hereinafter referred to as “color toners”) under the present situation because it is difficult to manufacture a transparent magnetic substance, and a color tint of a magnetic substance cannot be extinguished completely. In addition, when using color toners that use colored magnetic substances for full color development, there is a problem of a narrow range of color reproduction. Thus, the magnetic substances cannot be used for color toners under the present situation. On the other hand, the system using the magnetic carriers has problems of the complicated apparatus because, for example, a magnet for restricting the carrier is required, a sensor for measuring a mixing ratio of the carrier and the toner is required, or control means for controlling the mixing ratio is required.
Therefore, a developing apparatus, which can selectively furnish plural kinds of developers for development using a non-magnetic single-component developer as a developer, is desired.
As such a developing apparatus, as disclosed in Japanese Patent Application Laid-Open No. S55-43533, there has been proposed a method of injecting charge to a toner with a doctor blade using plural non-magnetic single-component developers (toners) having different charging orders. With this method, rather than selectively coating only one kind of toner among toners on a developing roller to be supplied to a developing part, the toners on the developing roller are supplied to the developing part in a mixed state and selectively furnished for development depending on a difference of charging amounts of the respective toners.
However, sufficient selectivity cannot be obtained by the method using a difference of charging amounts and causes color mixture in a developed image. An image failure called “muddled color,” in which a toner of a color which should not originally be supplied is supplied to an image part on an object to be developed, tends to occur. In addition, since a positively charged toner passes through the doctor blade to be furnished for development, an image failure called “fog,” in which the toner which should not originally be supplied is supplied to a non-image part on the object to be developed, tends to occur.
In order to cope with this problem, Japanese Patent Publication No. H2-37595 discloses a method of providing an electric field between a supply roller for supplying a toner onto a developing roller and the developing roller and selectively supplying the toner to the developing toner to cause the developing roller to bear (carry) the toner.
However, as described later, in the developing apparatus disclosed in Japanese Patent. Publication No. H2-37595, it is unavoidable to bring the supply roller and the developing roller into a non-contact state due to a problem of color mixing or the like. In addition, even in that case, if a toner as a non-magnetic single-component developer is used, since a mixed developer on the supply roller is charged only by a regulating member brought into abutment against the supply roller, a distribution of charging amounts is broad, the toner does not fly from the supply roller to the developing roller uniformly, possibly causing unevenness on the developing roller.
Consequently, Japanese Patent Publication No. H2-37595 recommends an arrangement in which a supply roller containing a magnetic developer and a magnet is used to cause the developer to fly from the supply roller to the developing roller uniformly under an alternating electric field.